Vibrational spectroscopy applied to the study of milk proteins

Vibrational spectroscopy is a versatile tool for analysis of foods including dairy products. This paper provides an overview of techniques that may be useful to study structural properties and interactions of milk proteins. Specific examples of recent research conducted in our laboratory using Raman, micro-Raman and FT-Raman spectroscopy will be described to illustrate their applications for investigating protein-protein, protein-lipid, and protein-polysaccharide interactions, and for elucidating structure-function relationships of whey proteins and peptides. Raman spectroscopy was used to discriminate between structural changes of beta-lactoglobulin in transparent fine-stranded versus particulate gels. Formation of particulate gel structures led to a more hydrophilic or exposed microenvironment around tryptophan residues, compared to a more hydrophobic microenvironment in translucent gels. A decrease in the alpha-helical content was more pronounced in translucent gels, while both types of gels retained considerable beta-sheet structures, consistent with the known heat-resistance of the beta-barrel structure of beta-lactoglobulin. Principal component similarity analysis demonstrated that heat treatment and heat-kappa-carrageenan interactions were the most influential factors affecting whey protein structure as monitored by spectral changes. Raman micro-spectroscopy was applied to investigate protein-lipid interactions at the interface between bovine serum albumin solution and mineral or corn oil. While Raman bands assigned to aromatic and aliphatic residues inferred involvement of hydrophobic interactions at the oil-water interface, the secondary structure was not significantly altered. Temperature-dependent changes in the Raman C-H stretching region of liposomes composed of phospholipids with varying headgroups and acyl chains were influenced by the presence of bovine lactoferricin. This approach may be useful to elucidate the interactions of cationic antimicrobial peptides with bacterial versus host cell membranes. A diverse range of other applications as well as new developments in techniques are emerging in the literature, indicating the potential for increasing use of vibrational spectroscopy in the analysis of milk and milk products.